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The good news is that when I changed my sparkplugs, the engine now runs smoother than ever at low RPM's. But the problem at high RPM's is still there. It won't accelerate smoothly after 2500 but it will stay running smoothly up to about 3000, above that, it runs rough and the performance is degraded. I can only reach 4600 at WOT and it usually gets 5600 when its running right. Its a 4-cyl Merc Big foot 50, non EFI.

Can any of you mechanics out there help me narrow down the problem? It started running poorly right after a tank change. The tank had a bad valve (now replaced) and may possibly have had some water in it due to a recent rainstorm. I've since tried switching tanks and that hasn't corrected the problem either. Anyone have experience with water in a carb bowl? Could it cause a problem like this? I figure either water or another carb problem (like clogged high speed jet) is the most likely cause...and the next thing I'll probably try to do is drain the bowls. I suppose I really should rebuild the fuel tank system and put in a water separating filter and 2-way tank valve.

The other possible causes I've thought of so far would be clogged engine fuel filter (I replaced the fuel line filter a few months ago and it looks good) or possibly an ignition problem in the coil or plug wire. Or worse, it could be a sticking valve problem. My intuition tells me that its less likely to be an ignition or valve problem when the engine runs so well at low RPM's though.

Anybody had similar problems and/or have some advice?

Dimitri,
It sounds to me like you have a clogged high speed jet. I had the same problem this summer. I rebuilt the carbs over the winter and it works fine now. The mechanic told me to try this trick or test. Start the engine and run it up under load. This is to be done in the water if possible. It won't work without a load. If you pump the fuel bulb and it still stays running it is the fuel pump. If it still does not run right it is the jet. To drain the water out of the bowl there should be a screw at the bottom of the bowl to drain the fuel out. A rebuilt kit should be pretty easy to do. I did all three of my carbs in about three hours in 40 degree weather. That was taking my time to watch a little tv and warm up at the same time. As a side note the motor ran fine in the driveway. When I put it under load that is when I had problems. I hope that helps. Happy sails. Tony

I added a separate inline fuel filter on mine.

Sure sounds like dirt or something in the fuel jets. I leave the vents on my fuel tanks open all the time and no problems with water yet. The tanks are fairly well sheltered in the fuel lockers. Make sure the tank vents are open while the engine is running at least. I've remeber one guy coming back to his slip complaining about engine performance on his nearly new motor and the tank vent was closed and the fuel tank was halfway collapsed. Even after I explained it to him, he said that his dealer told him to keep the vents closed and he would not open them. Oh well.

I would also check the ignition timing. Not enough advance would run great and super smooth at low speeds but would not perform well at high speed.

Also check for air leaks into the fuel system at all connections including the fuel filter that was removed.

Tony,

That's what I'm afraid of, but I figure I'll try the easy stuff first. I got a new engine fuel filter yesterday and should have it installed in the next day or two. I'll have to pull the boat out if I need to dissassemble the carbs, wouldn't want to do that on the water. Mine is 4 cyl with 4 carbs..or maybe its just one 4-part carb but it definitely has a fuel bowl, adjustments, jets, etc. for each cylinder. I don't think I've done that sort of job since I was a teenager working on a good ole fashioned muscle car. I wonder if you need any sort of special tools to adjust all carbs in proper harmony. I guess that would be a good reason to get a service manual first.

Craig, I put a new fuel line on it a couple months ago because the bulb on the old one went bad. I cut another inline filter into the new hose too although I think the way to really go is those canister filters with the clear (observervation) bowl and drain. My inline filter in the engine compartment looks like the original one by virtue of the factory looking plastic hose ties. I think most people put proper SS hose clamps on it the first time its changed. So, hopefully, I'll get lucky and thats what it is. If it is really obstructed, I don't know if you could put enough pressure on it with the primer bulb to make any sort of difference. Unfortunately, the boat had been sitting in a grassy storage lot for at least 6 months when I bought it last June and the 4 tanks that came with it were pretty dirty. I did kind of a half baked job of cleaning them out...probably should have been more diligent in hindsight....although it ran fine for 6 months before this happened.

Last weekend, I tried to put my old fuel hose back on and that made it run even worse, I probably added yet more contaminated fuel to the mix since that hose hadn't been used in 2-3 months. Even with the bad bulb, the line still primed, I guess a Mac is good for that since the tanks sit higher than the engine. Who knows, we may not really even need primer bulbs on these boats! Anyway, after I put the new hose back on, it was running rough even at low RPM's for a good 10 mins, then finally, it settled back into what it was doing before (as I nervously got back to my dock). These engines seem to be very sensitive to fuel flow issues. I had a Mercury SportJet 175 (2stroke inboard on a jetboat) for about 5 years without ever having an engine problem. It had a built-in 25 gal tank...I'll bet those tanks don't even drain from the bottom (ie. unusable fuel always remains) which would mean that any dirt would stay in the tank.

Thanks for the responses and I'll keep you posted.

On mine I had air leaks into the fuel line and it made the motor very hard to start and it would die when lef to idle for more than a few seconds.
The fittings at the top of both fuel tanks were leaking air in.
I think I brushed soapy water over the fittings and shook the fuel tanks with the vents closed to find the leaks, but it has been several years and don't remember exactly.

Also worm drive hose clamps often need to be retightened a day after installation for some reason. They tend to become loose when newly installed. Maybe the rubber compresses and makes them loose.

Usually there is a screen filter on the dip tube inlet in the fuel tank. then the inline filter you installed, then the trash filter on the engine itself. If all these filters are in place, I am tending to think maybe dirt is less likely unless it got in before the inline filter was installed. Old gummed up gas might still be a possibility if the gas aged more than a year.
With solid state ignition, the first three things I look at to troubleshoot now are the fuel system, the fuel system, and the fuel system.

Good luck.

dimitri, seems that you are looking younger than ever now! really pretty the new photo !

regarding the 4 synchronized carbs... don't touch!!! i have the yamaha 50hp 4 stroke which is imho the same engine as the mercury. i had a problem that the engine died at idle when hot (very helpful when navigating with crosswind and current in a narrow marina !). the dealer only could fix it after purchasing himself a special tool for the carb sync.

Norbert, thanks! Being a Mac sailor will keep you looking young but can also make you lose some teeth!

I am also wary of changing any individual carb adjustments and so far have only adjusted things that affect all four (like throttle linkage, etc.). I wonder if it is possible to clean out the carbs without upsetting any of the adjustments?

Craig, did you see any bubbles anywhere when you got the air leaks? I have a clear inline filter on my fuel hose and it appears that there are no bubbles. At slow speed, the filter stays mostly full of fuel, but at high speed, it empties out and you can see the fuel coming in from the tank side....but thats what it used to do before I had troubles too.

are you shure? i thought that thoses filters have always to be full with fuel. could this be the reason for your problems? a clogged fuel line or tank outlet?

norbert wrote:are you shure? i thought that thoses filters have always to be full with fuel. could this be the reason for your problems? a clogged fuel line or tank outlet?

Well, anything is possible and perhaps its worth the investment of yet another new fuel line, but I did try to switch between two fuel tanks which both have new valves now, and I also tried a fuel line which still worked 2-3 months ago. Even though the primer bulb is bad on the old line, it still primed and the filter filled with fuel.

I'm just going to try systematically cleaning everything up, starting with the engine fuel filter. I'll probably try completely removing the 4 bowl drains and pumping some fuel through with the primer bulb to flush the bowls as much as possible...then perhaps squirting some carb cleaner in before putting the screws back. Of course, this may not help if a jet is plugged...but its worth a try. If all that fails, I can't think of anything else to try short of pulling the boat out and rebuilding the carbs.

While we are on the subject, anyone know of a good way to clean gas tanks? I've been trying to dump out the black crud with the last bit of fuel everytime I drain a tank lately, but its very hard to get that last few ounces out of the tank and so some always remains. I've started trying something new which is to leave the tanks upside down (on top of a 5 gallon bucket) until they are completely dry, Maybe this will force the sediment to stick to the top of the tank and not bother me anymore

One thing thats for sure, if I go to the trouble of rebuilding the carbs, I need to make sure the rest of the fuel system (tanks, hoses, filters, etc.) are completely clean. I gotta get this resolved before prime sailing season hits in a few weeks

Dimitri,
If you are going to clean the carbs get a carb kit for the motor. I did this and the floats for mine came preset. I could not change the float level if I wanted to. A lot of brake cleaner and reinstalling the new parts was all I did. I have three carbs on my Nissan 50. I just removed and replaced. In your carb rebuild kit it should have new floats seals and gaskets for all the carbs. I found my clog on the top carb for my engine. After seeing all the goo in the jet I now know that I must empty the fuel line after after every run to make sure that I don't have to go through that again. I especially don't want to go through that once I get he Suzuki 140. I would check the carbs first. I tsounds like a clogged jet to me. Enough said by me. Happy sails. Tony

Dimitri,

In getting the crud out of the gas tanks, why not vacuum it out with a siphon hose attached to a stiff wire. As the fuel exits the hose, let it run through a filter cloth into a clean container. Before you start, place the tank on its corner so all the foreign stuff settles in one area. You could repeat this several times with the same fuel as it becomes cleaner each time.

My fuel line air leak occurred at the top fitting that exits the fuel tanks. A little 5200 stopped the air leaks. Not sure if this was the right way to fix it, but it worked for 3 years now. I think I have the same inline clear filter. Before priming, there is lots of air (or vapor) in there. After priming only the occasional small bubble shows up. Certainly should be mostly fuel when running full throttle. Sounds like you have air leaking into the hose. Probably at the same spot. The quick connects should also be checked to make sure they are clean and connecting securely. If you have a bunch of air in the fuel filter sounds like you might have found the problem.

Thanks for the suggestions, I got a siphon pump hose today and will try to clean all the crud out tomorrow...sounds like a great idea to clean out the tanks. I replaced the engine fuel filter this evening and it was really gummed up on the inside, but it still didn't fix the problem. If I rev it up in neutral, it runs up to high rpm pretty well, but as soon as there is a prop load on it, it runs real bad, and now seems even worse as it won't get much over 2000 RPM anymore. I ran it like that for about 5 minutes and then stopped it and pulled the sparkplugs out. The top two cylinders were kinda gummy and wet fouled whereas the bottom two were white and dry. I need to troubleshoot the ignition system a bit more before deciding to rebuild the carbs, but the fuel system is definitely gummed up so I suppose that is still the most likely cause. I wish it was some air in the line, but I think its just running too badly now for that likelyhood. Who knows, may only be running on two cylinders now although I would think that would make it have a pretty rough idle, which is still fairly smooth.

Dimitri,
After what you said about the engine running fine without a load it points to the carbs. Mine did the same thing. The ignition system is working fine because it runs well in neutral. If it did not then you could point to it. Break down and buy the carb rebuilt kit. Brake cleaner also. Happy sails, Tony

Dimitri. This is a long story. I think our model of the Mercury must have a pretty crappy fuel system.

I have a year 1999 4-cyl Mercury carbeurated 50-HP, non-bigfoot. Same engine as yours, different lower unit. I'm located in St. Petersburg and keep the boat in a slip. I run the same fuel as everyody else at the marina, keep the tanks free of water, and so forth, but I have had fuel system problems.

FIRST TIME was a stuck float --- at low speed the engine ran badly and fuel would overflow carb when at idle. The problem cleared up by itself sometime during a long motor cruise to visit a mechanic friend in Gulfport who was going to take a look at it for me. Cost me only a six-pack and a lot of kidding about "non-problems".

SECOND TIME was similar to your description --- fine at all speeds in neutral; but under load started stumbling at about 3500 rpm and wouldn't run over about 3800 rpm. Tried changing plugs, spraying carb cleaner up into the air intake of the silencer, fuel system cleaner in the gas, etc. Nothing worked.

Advice from mechanic was that it was clogged high-speed jet(s) and would need to go to shop to rebuild the carbs. I was going to try doing the carbs myself. I have the service manual. Discovered that this is quite complicated procedure, because the whole 4-carb and silencer assembly must come off the engine as a unit, then disassemble, then re-build carbs, then reassemble, then reattach 4-carb assembly to engine, then use a special (expensive) tool to re-sync the carbs, then remove the 4-carb assembly, then attach silencer, then re-attach the whole friggin works to the engine and adjust the overall linkage. Decided it was beyond me.

An engine mechanic on the rec.boats newsgroup suggested I might try the following before taking it in. "Run the engine out of fuel, fill the carbs with Mercury Powertune, let it sit overnight, drain the carbs, then give it a try." He said there was an only an outside chance that would work, but cheap enough to give it a try. He said to be sure to use the Powertune and not some other cleaner.

Problem is that there's no way to get to the top of the carbs to put in the Powertune because of the silencer unit which cannot be separately removed while carbs are on the engine (as mentioned above). What I wound up doing was (after running engine out of fuel) squirting the Powertune up through the carb bowl drains to fill them, using a finger to keep it from leaking out while replacing the plug without loosing much. Messy and lots of wasted Powertune. (Definitely a no smoking job.) Let it sit a day and a half, drained the carbs, tried it. It WORKED. Engine ran great at full power. Got LUCKY.

THIRD TIME, same symptoms as above although not quite as bad. Engine would get up to maybe 4000 rpm but no real power. Returned to the slip without sailing. I was pretty disgusted to say the least. I added some fuel system cleaner to the gas tank just for the hull of it, tilted the engine up, hooked up the water hose for fresh water flush, and started the engine, running at probably about 2000 rpm. While pi$$ and moaning to a fellow in a slip down the way about the engine problems, I unintentionally left the engine running that way for a long time, about 45 minutes, maybe an hour. On a whim I decided to take her back out again and at least get in a little sailing that day. The engine ran great, full power. I have no idea whether it was the fuel treatment, flushing the engine so long, or running it so long in the tilted up position, or some combination of those. Got LUCKY again.

After that I talked with the mechanic on rec.boats who had given me advice before about filling the carbs with Powertune. Asked "Why am I having so much trouble". We determined that some possibilities were:

a) I hardly ever opened up the throttle on the engine; just slowly motoring in and out in displacement mode. Quiet and relaxing, arm around my girlfriend, enjoy the setting sun after a great day of sailing, life is good, an so forth. His advise was to use full-throttle for a while each and every trip, especially on the way back in. Yech!

b) I sometimes go several weeks without using the boat. Advise was to run the engine out of fuel if it was going to sit for a while, especially during the summer here in FL. There was much argument from other people on the newsgroup that this "shouldn't be necessary", "was stupid" and so forth. You know the scene.

c) I seldom fresh-water flushed the engine, mainly because upon returning to the slip the crew always seems in a big hurry to "get going" somewhere. Advise was to always flush -- that salt in the engine can cause hot spots that can affect the fuel system. Once again there was much argument from other people on the newsgroup about this, that "no way could this affect fuel system" and so forth.

Since then I _religiously_, winter or summer, each and every time:

1. Use full throttle for a good period of time every time out. And always for the last part of the trip coming back to the marina. Don't like it, but I do it.

2. Upon returning to slip, disconnect the fuel line, tilt the engine, hook up fresh water, start the engine and let it run until it runs out of fuel. Takes about five minutes to run out of fuel, plenty time to fresh water flush. Maybe twenty minutes total. If crew are dancing around in a hurry to go, that's just too bad. "Why don't you go ahead to the restaurant and I'll meet you there." or, if only one vehicle, "Please wait for me up at the marina office.".

I've had no more problems for a year and a half. Yeah! (knocking on wood).

A carb job will be quite expensive, so you may want to first give the above things a try:

1) Add a fuel system cleaner to the gas, tilt the engine up, attach fresh water, let the engine run at "higher than idle" speed for an hour or so.

If that doesn't work,

2) Drain the carbs and squirt the Mercury Powertune up into the carbs through the drain holes. Try your best to get them all the way full. Let it sit a day or so. Then drain the Powertune out and try the engine.

I hope you are as lucky as I was.

Chuck H.